I have never heard of cam gear teeth shearing off. There is an oiler on the front plate of the engine that could possibly get clogged. I would also guess that the cam could be seized or for other some reason stopped. To shear the teeth, the crankshaft gear would have to be turning. I would recommend a good internal cleaning of the engine. While it is apart, drop the pan and clean out the sludge in the bottom. When and if the shop gets it running use one of the several techniques suggested here to run a cleaner through the engine while running. I would check to see if you have oil to the rockers, if not spray some cleaner into the supply tube, let it sit for a while and use a little air to blast out any blockage.
The decision to replace or not should be made after a compression check after the internal cleaning. You may also want to run some top cleaner through the Carb to do some clean up in the head. My guess is that you have to make a decision to rebuild or replace, just replacing parts without figuring out the cause is not going to work out for you in the long run. It also depends on your goals for the truck. If you decide to replace, make sure your new engine will fit into the engine compartment and link to your drive shaft, and other components.
